The employment of alternative fuels such as liquefied natural gas (LNG) to power automotive vehicles has provided advantages that are both economical and environmental compared to conventional hydrocarbon fuels such as gasoline and diesel. Accordingly there is a growing demand for vehicles that are fuelled exclusively by LNG, and for bi-fuel vehicles whose engines are capable of fuelling from gasoline or diesel in addition to LNG.
Gasoline and diesel are incompressible liquids and accordingly, unlike gaseous fuels their densities do not change as a function of pressure. The heating value of gasoline or diesel is substantially constant and orthogonal to pressure. A measurement of the volume of gasoline or diesel remaining in a fuel tank provides sufficient information to determine the energy content available to power a vehicle. This is not the case for LNG.
Measuring the volume of LNG provides a vehicle operator with information they can use to estimate when a trip to a refuelling station is necessary. However, the density of LNG is a function of temperature, pressure and fluid composition, and therefore the energy content in any given volume is variable. At the pressures and temperatures that LNG is typically stored, it is a multiphase fluid, and the density of the liquefied gas is variable as a function of the saturated pressure and temperature. A measure of the liquid volume in a fuel tank provides less accurate information to estimate the energy content available to power a vehicle, in comparison to gasoline or diesel. For example, under typical storage conditions the density of LNG in a cryogenic storage tank can vary up to 20% or more. The vapor pressure within a cryogenic storage tank changes due to boiling and natural expansion of the LNG as a result of heat leak into the cryogenic space from the external environment because of the finite thermal resistance of tank insulation. For a given mass of LNG, however, the energy content available to power a vehicle is the same even though its volume and density can vary by 20%. Therefore determining the mass of LNG provides a more accurate estimation of its energy content and therefore fuelling range of a vehicle.
It is known to use level sensors to measure a liquid level of LNG in a cryogenic storage tank. However, accurately measuring the liquid level of a cryogenic liquid held in a storage tank is still a challenging application. It can be especially challenging to accurately measure liquid level of cryogenic liquids in storage tanks that are mobile, such as vehicular fuel tanks storing LNG. There are known methods available for determining the liquid level of a cryogenic liquid held within a storage tank that employ level sensors. There are various types of level sensors including mechanical float-type level sensors, pressure-based level sensors, ultrasonic level sensors and capacitance-type level sensors. It is known to use a capacitance-type level sensor for measuring liquid levels inside a cryogenic storage tank.
The capacitance-type level sensor has proven to be particularly well suited for measuring the level of LNG in a storage tank. The evolution of capacitance-type level sensors has provided sensors of varying complexity and accuracy tailored to particular application requirements. The basic operating principle behind a capacitance-type level sensor is to arrange two conductors within a tank where the liquid level is to be measured. The conductors are electrically insulated by a space that provides for a dielectric material. That is, the LNG between the conductors in liquid or vapor form serves as the dielectric material. The combination of the conductors and the dielectric material therebetween provides a capacitor. A capacitance of the capacitor is directly proportional to the surface area of the conductors, the distance separating the conductors and an effective dielectric constant of the dielectric materials between them. As the level of liquid rises or falls within the tank, the effective dielectric constant of the dielectric between the conductors changes and so too does the capacitance. By detecting changes in the capacitance of the capacitor the level of the liquid in the tank can be determined.
For the purposes of this application, cryogenic fuels include those liquid fuels that boil at temperatures at or below −100° C. under atmospheric pressures. For example, LNG boils at approximately −162° C. at atmospheric pressure. While the present description pertains to LNG, it is equally as applicable to other multiphase fluids generally, for example methane, ethane, propane, hydrocarbon derivatives, hydrogen, nitrogen, argon and oxygen.
Accurately detecting the level of liquid remaining for vehicular fuel tank applications is important because the consequence of an inaccurate level measurement can result in a vehicle being stranded if it runs out of fuel, or reduced operational efficiency if the vehicle is re-fuelled more frequently than necessary. In addition, for vehicles that use a high pressure pump to deliver the fuel to the engine, there can be accelerated wear of the pump components if the pump is operated frequently when the fuel tank is empty.
During refuelling of a cryogenic storage tank an ullage space needs to be provided for natural expansion of vapors from boiling of the cryogenic liquid. One of the challenges of LNG is that, in many applications, once delivered into the storage tank, it needs extra space in which to expand when the LNG warms. Excessive heat leakage into a cryogenic tank, as well as causing the LNG itself to expand, will cause the cryogenic liquid to boil. Eventually, with continued heat leakage, LNG will boil or evaporate resulting in a pressure build up in the storage tank.
One problem with use of the ullage space is that it is difficult to leave an adequate space during filling. In other words, refuelling must be stopped at some predetermined point prior to the storage tank reaching liquid full. Ideally, the ullage space should be large enough to allow for LNG expansion yet small enough to maximize the amount of cryogen that can be held in the tank and, thereby, maximize the time between refuelling. As noted above, this is important in natural gas vehicle operations where fuel systems attempt to maximize the volume they can store within the limited space available on a vehicle while minimizing the space utilized to store that fuel. A variety of means have been developed to determine a fill point that leaves an adequate ullage space.
Visual fill lines, if used, may not provide the level of accuracy required. Also, given the double-walled, vacuum insulated structure of many tanks, it is not easy to provide a sight port through to the inner vessel. Stop mechanisms such as shut-off floats or valves require mechanical parts within the inner vessel. This introduces into the storage tank a mechanical failure point that is subjected to wear during and between each fill.
Level sensors have been used in cryogenic storage tanks. However, in order to calibrate the level sensor the storage tanks have traditionally required filling which is problematic for a number of reasons. First, it is difficult to achieve when an ullage space is required and no visual fill lines are present. Second, filling of a storage tank during manufacturing is not desired since the tank then needs to be emptied after calibration for shipping. Also, once the level sensor is assembled into the tank it is difficult to gain further access if required for calibration purposes.
U.S. Pat. No. 6,892,572 issued to Breed et al. on May 17, 2005 discloses a system for determining a quantity of a liquid in a fuel tank in a vehicle subject to varying external forces caused by movement or changes in the roll and pitch angles of the vehicle wherein the tank is mounted to the vehicle and subject to forces along the yaw axis of the vehicle. One or more tank load cells provide an output proportionally representing the load thereon. The load cells are placed between a portion of the tank and a portion of a reference surface of the vehicle and are sensitive along an axis that is substantially normal to the mounting surface and generally parallel to the yaw axis of the vehicle.
French Pat. No. 2,885,217, issued to Bruno Bernard on Aug. 10, 2007, discloses a gaseous fuel quantity measuring gauge for a fuel tank of a vehicle comprising sensors measuring pressure and temperature of fuel within the fuel tank. There is also disclosed a method and apparatus for estimating the mass of a liquid disposed below the gaseous fuel. Bernard teaches an arrangement, which employs multiple sensors that introduce further heat paths between the storage vessel and the outside environment, increasing the boiling rate of the liquid within the vessel. Furthermore, Bernard disclosed that the pressure sensor must be disposed at the bottom of the tank in an inconvenient location for mounting, thereby complicating the manufacturing of the storage vessel and introducing an additional failure point that either shortens the operational life of the fuel tank or increases the maintenance costs.
A fluid level sensor employing multiple stacked capacitive sensors is disclosed in U.S. Pat. No. 3,797,311 issued on Mar. 19, 1974 to Blanchard et al. The fluid sensor comprises a lower segment, an intermediate segment and an upper segment. When a level of fluid is within the range of the upper segment, the lower and intermediate segments do not contribute at all to the level measurement. Instead, a fixed height of the lower and intermediate segments measured before installation is added to the output of the upper segment.
A capacitive level sensor and control system is disclosed in U.S. Pat. No. 6,016,697, issued Jan. 25, 2000 to McCulloch et al. The capacitive level detection and control system provides a highly accurate determination of liquid level within a container. The primary sensor is an elongate capacitive probe positioned vertically within the container so that a lower portion of the probe is in liquid and an upper portion of the probe extends above the surface of the liquid. A liquid reference sensor is proximate the lower end of the probe, and a gas reference sensor is proximate the upper end of the probe. The gas reference sensor and the liquid reference sensor assist in calibration of the system and provide capacitances proportional to liquid and gas dielectric constants. The calibration requires that all three sensors be placed in the same medium, for example air, so that voltages can be measured. During operation, then, the level measurement is independent of the dielectric constant of the liquid whose level is being measured.
There is a need for a new and improved apparatus and method for determining the volume and mass of a multiphase fluid that employs a level sensor measurement.